Ecosystem effects of red king crab invasion -a modelling approach using
Ecopath with Ecosim
by
Jannike Falk-Petersen
Master Thesis in International Fisheries Management FSK 3910
Norwegian College of Fishery Science University of Tromsø
May 2004
ACKNOWLEDGMENTS
I wish to thank Prof. John Field at the University of Cape Town and Dr. Lynne Shannon at Marine and Coastal Management, Cape Town, for helping me in the initial phase of my thesis. Special thanks to Lynne Shannon for helping me to understand the use and limitations of Ecopath. Thanks to Prof. John Pope for good discussions and getting me in touch with Drs. James Ianelly and Kerim Aydin at the Alaskan Fisheries Science Center who gave me very useful information related to their work on the red king crab and ecosystem models.
I would also like to thank my supervisors, Drs. Torstein Pedersen and Einar Nilssen, for feedback on my work and nice cruices to Ullsfjorden, Dr. Tore Haug at the Institute of Marine Research for information on seal diet, Raul Primicerio for good discussions and Frøydis Strand for supply of map and king crab figure.
SUMMARY
Knowledge on effects of the invasive red king crab (Paralithodes camtschaticus) on the Barents Sea ecosystem is limited. Due to the information available on benthos the Ecopath model of Sørfjord, Northern Norway, was used to investigate possible trophic changes with introduction of king crab to the model. A literature study of the king crab was conducted to find information on diet, mortality, consumption rate and other life history parameters required by the model. A short introduction to biological invasions was also included. The Ecopath with Ecosim software was used as a modelling tool to investigate factors allowing for growth of king crab as well as potentially controlling their biomass. Possible food-web effects of king crab invasion on the Sørfjord ecosystem was also investigated. Knowledge gaps identified through the exercise and management implications were discussed. A biomass of 1.2 t km-2 small and 2.8 t km-2 large king crabs was introduced to the Sørfjord model. The modelling exercise indicated that a resource opportunity, in the form of benthic production, could allow for growth of the king crab in Sørfjord. Mammals could have a negative impact on large king crab abundance through predation, while fish predation is expected to have minor effects on king crab biomass. King crabs are expected to have a negative effect on the benthic community through predation, but limited impact on the pelagic community. King crab abundance could be controlled by fishing large king crabs. A change in fishing of other commercial fish species is not expected to have an impact on king crab biomass.
Knowledge gaps identified from this exercise includes population structure, biology and food preference of king crab. Production and interactions within the native benthic community should also be investigated further to understand possible impacts of king crab invasion.
TABLE OF CONTENTS
1. INTRODUCTION... 3
1.1. OBJECTIVES ... 3
1.2 BIOLOGY OF THE RED KING CRAB... 7
1.3 BIOLOGICAL INVASIONS... 13
1.4 ECOPATH AND ECOSIM ... 17
2. MATERIALS AND METHODS ... 19
2.2 CONSTRUCTION OF THE SØRFJORD MODEL WITH KING CRAB ... 21
2.3 ECOPATH OUTPUT ANALYSIS... 31
2.4 ECOSIM MODEL INPUTS ... 32
2.5 ECOSIM MODEL RUNS... 33
3. RESULTS ... 35
3.1 ECOPATH MODEL OUTPUTS ... 35
3.1.1 Mixed trophic impact analysis ... 39
3.1.2 Sensitivity analysis... 39
3.2 ECOSIM ANALYSIS... 42
3.2.1 Run II A ... 42
3.2.2 Run II B... 43
3.2.3 Run II C... 45
3.2.4 Run II D ... 51
4. DISCUSSION ... 52
4.1 UNCERTAINTIES OF INPUT VALUES ... 52
4.2 LIMITATIONS OF MODEL ... 54
4.3 WHAT FACTORS COULD CONTROL KING CRAB ABUNDANCE? ... 56
4.3.1 Availability of resources ... 56
4.3.2 Fishing... 59
4.4 WHAT EFFECTS WILL THE KING CRAB HAVE ON THE SØRFJORD ECOSYSTEM AS PREDICTED BY ECOPATH WITH ECOSIM? ... 63
4.4.1 The positioning of king crab in the food web ... 64
4.4.2 Effect of king crab on benthic groups... 64
4.4.3 Effects of king crab on benthopelagic and pelagic groups ... 65
4.5 WHAT ARE THE IMPLICATIONS FOR MANAGEMENT? ... 67
4.6 WHAT FACTORS HAVE THE MODELLING EXERCISE IDENTIFIED THAT WE NEED TO LOOK FURTHER INTO?... 68
4.7 CONCLUDING REMARKS... 69 5. REFERENCES... 71 APPENDIXES ... 83
1. INTRODUCTION
1.1. OBJECTIVES
During the 1960s Russian scientists introduced the red king crab (Paralithodes
camtschaticus) to the Barents Sea. Over a period of 10 years king crabs were caught mainly in the Sea of Japan outside Vladivostok and released in the Kola fjord. The motive of the introduction was to improve the coastal fishery economy as the crab is a highly valuable commercial species (Orlov and Ivanov 1978). Since the introductions the crab population has grown and expanded its distribution down the Norwegian coast (Figure 1) (Jørgensen et al. in press).
Figure 1. Generalised distribution and spread of the red king crab from area of release (read) and westward expansion (Sundet 2002).
The Norwegian government is working towards ecosystem management of the Norwegian marine resources. An emphasis is put on sustainable management of the ecosystem
securing social and economic interests including the possible threat of introduced species.
Norway also has high ambitions when it comes to following up international treaties including those concerning protection of biodiversity (Anonymous 2002). An overview of
Rybachi Island
Kola Peninsula
Norway
Russia
Varanger fjord Tanafjord
Laksefjord Porsanger
Sørøya
North Cape Hammerfest
the many treaties, conventions and instruments of relevance to Norway concerning alien species is to be found in Hopkins (2001).
The red king crab is an invasive species in the Barents Sea. Being a large, bottom feeding omnivore of high mobility it may be hypothesised that it will have a significant impact on the ecosystem.
The focus of research has until today been on population biology, distribution and modification of harvesting tools to reduce bycatch of king crabs in traditional fisheries.
Diet, temperature tolerance, parasites and symbionts have also been studied as well as the effect of king crab on scallop beds. The management plan of king crab in Norwegian waters expresses its concern regarding the consequences of the crab as an introduced species. This has been followed up by the Institute of Marine Research by focusing on ecosystem effects of king crab. The future focus of research will be on the biology of king crab, their impact on survival of capelin eggs and effect of king crab on the existing habitats and consequences for species interactions (Godø et al. 2003).
While there have been a number of speculations, knowledge on the effect of king crab invasion on the Barents Sea ecosystem is limited. There is a lack of information on what was there prior to invasion and the understanding of factors structuring the ecosystem is restricted (Godø et al. 2003). The possible ecological consequences of king crab invasion is an important factor in the debate concerning how Norway should manage the Barents Sea ecosystem.
Ecosystem analysis of the Barents Sea has so far focused on the pelagic species (Bax et al.
1991, Sakshaug et al. 1994, Tjelmeland and Bogstad 1997, Tjelmeland and Bogstad 1998, Dommasnes et al. 2002), while data on benthic communities is limited (Nilsen 2001).
Ecopath with Ecosim (EwE) is a software for constructing and analysing trophic flows in a system as well as exploring impacts of fishing and environmental disturbances. While Ecopath can be considered an analytic tool, Ecosim can be used to predicting changes in abundance of system components over time. Ecosim has successfully been used to predict
changes caused by fishing in the Gulf of Thailand (Christensen 1998) as well as rejecting trophic interactions as the main force causing structural changes of the Bering Sea from 1950s to 1980s (Trites et al. 1999, Christensen and Walters in press).
Based on Ecopath the Sørfjod model was constructed (Pedersen, T., Nilsen, M., Nilssen, E.M. and Berg, E. unpubl.). Due to the information available on benthic production (Nilsen 2001) the Sørfjord model is useful as a base looking at possible trophic changes associated with king crab invasion. The Sørfjord is also a relatively closed system making it well suited for ecosystem analysis with an Ecopath type model.
In this study an extensive literature review was conducted to find the input data required by the model on diet as well as mortality, consumption rate and other life history parameters of king crab. The biology of the king crab is initially described as it is an important key to predict possible effects of the crabs on an invaded system. A short introduction to the literature on invasive species will follow giving a presentation of some of the issues of concern with respect to invasive species.
Ecopath has previously been used to compare a system before and after invasion (Moreau et al. 1993), but is most commonly used to evaluate past and future effects of fishing. In this work, however, Ecopath and Ecosim will be used as a modelling tool to investigate possible trophic impacts of king crab invasion as well as identifying factors that could control the king crab population. Due to the relatively comprehensive benthic data material of the Sørfjord model an attempt to quantify the importance of bottom-up and top-down control can be made. The strength of these links can further indicate if the effects of king crab invasion will be limited to the trophic levels it feeds on or cause significant changes at higher trophic levels. It can also give an indication of the potential of higher trophic levels to control king crab abundance. Identifying factors controlling king crab abundance can help explaining its success as an invader. The output of the Ecopath and Ecosim analysis will be used to determine what groups in the system may be most vulnerable to king crab invasion.
The use and limitations of Ecopath with Ecosim for investigating ecosystem consequences of king crab invasion will be evaluated. Implications for management of the king crab based on the outcome of the modelling exercise will also be discussed. The knowledge gaps that may be important to fill in order understand ecosystem effects of king crab invasion as identified through the modelling exercise will be pointed out.
The objective of this thesis is to identify factors potentially controlling king crab biomass as well as factors allowing for their growth. Possible food-web effects of king crab invasion on the Sørfjord ecosystem will also be investigated. The implications of the findings for management as well as knowledge gaps identified through the analysis will be discussed.
1.2 BIOLOGY OF THE RED KING CRAB
King crabs (Family Lithodidae Samouelle) are among the world’s largest arthropods of the genus Paralithodes (Martin and Davis 2001, Zaklan 2002).
The red king crab has been recorded at a wide range of depths from intertidal and shallow rocky habitats of about 4 meters to about 510 meter depths (Rodin 1989, Klitin and Nizyayev 1999). Temperature tolerance has been recorded to range from -1.7 to +18ºC with an optimum of +2 to +7ºC. The crab has been found in water of salinities of 28-30 ppt and higher (Orlov and Karpevich 1965, Rodin 1989). Experiments of incremental exposure to dilute seawater showed that adults are less tolerant to low salinities (12ppt) than
juveniles (10ppt). Adults also showed poor performance in volume regulation and recovery (Thomas and Rice 1992).
The habitat of the mature king crab is determined by a mating-molting and a feeding migratory pattern. In late winter/early spring the crabs migrate shoreward to reproduce while in winter they move to deeper waters to feed (Marukawa 1933, Stone et al. 1992).
There is not agreement in the literature as to what govern the feeding migration. Stone et al.
(1992) point out that food availability is less in deeper waters. They found that photoperiod was well correlated with depth distribution. Temperature and salinity could also regulate movements especially during summer when shallow waters hold unfavourable
temperatures and salinities. Large-scale movement is normally undertaken as a group. The crab has been recorded to move over 10 kilometres in a day (Marukawa 1933). A tag and recapture study by Hayes and Montgomery (1963) found king crabs 110 miles from the point of release. In areas of large variation in depth migrations may be limited as suitable habitats and environmental conditions are available within a small geographical area (Wallace et al. 1949).
At about the age of 5 the king crabs reach sexual maturity. Otto et al. (1989) determined size of maturity for females in different areas to range from 65.7 to 105 mm carapace length (CL). Rafter (1996) found CL at maturity to be 100 mm for females and 108.2 mm
for males in the Varanger fjord, Norway. A practical average of 100 mm has been suggested by Powell and Nickerson (1965a).
When the crabs reach 5 years sexual segregation starts to emerge. In summer mature males move into deeper waters, while females remain in shallow waters. Mature females stay in water near 4 ºC, presumably to ensure optimal temperatures for the eggs to hatch prior to spawning. The males on the other hand, conserve energy in waters near 1,5ºC. Migration data of ovigerous females in Auk Bay, Alaska, indicates that from mid-June through mid- November the crabs aggregate and feed in relatively deep waters (mean depth 52.6m) below the summer thermocline. When the thermocline breaks down the crabs migrate to intermediate depths (mean depth 27.5m) where they release eggs fertilized in the previous spawning season. In May the females move to shallow coastal areas (mean depth <25m) to molt and mate. The males will join the females in shallow water where they clasp and guard females for up to 16 days prior to spawning (Stone et al. 1992, Stone et al. 1993, Loher et al. 1998). Spawning crabs prefer kelp areas where Alaria, Costaria, and Laminaria are common probably because they provide protection to the female during ecdysis (Powell and Nickerson 1965a). After spawning the adult crabs migrate back to deeper waters (>40m) (Stone et al. 1993).
The larvae hatch in early winter and spring and pass through four zoeal stages and a glaucothoe (the last larval phase in crabs) that settle and metamorphose into the first benthic instar (Nakanishi 1985, Paul et al. 1989). The inter molt period is influenced by water temperature with full larval development requiring an average of 469 degree days.
The intermolt period is 9 days at 8 ºC and 24 days at 2 ºC, while growth is impaired at temperatures above approximately 10 ºC. Experiments suggest that steadily increasing sea surface temperatures through the planktonic phase results in the most rapid development.
This is because the later stage zoeae perform better at higher temperatures than early stages (Nakanishi 1985). In addition to high temperatures other factors known to affect mortality rates of king crab larvae include stormy conditions and prey availability (Paul et al. 1979, Ishimaru 1936 as in Paul and Paul 1980).
Settlement occurs mainly in late July and August in near shore habitats. Characteristic habitats include cobble to boulders, shale outcroppings and biogenic structures including filamentous bryozoans, erect colonial ascidians and sponges, tibicolous polychaetes, mussel beds and filamentous algae. These structures provide refuge from predation a well as food (Powell and Nickerson 1965b, Sunberg and Clausen 1977). At this stage the king crabs 1 to 12 months old are 2.5 to 12 mm in carapace length (CL) respectively. Crabs 9 to 19 mm are commonly found on barnacle encrusted dock pilings between rays of starfish. It is believed that the crabs feed upon food particles dislodged by their commensal hosts (Powell and Nickerson 1965b).
The crabs leave their hidings between 12 and 24 months of age and form pod communities consisting of up to about 3000 individuals. The pods are believed to serve as protection against predators and provide biological organization and control. Pods have been observed from December throughout September. They consist of crabs of 17 to 69 mm CL, which are crabs from 2 up to 3-4 years old. The pods are disbanded either to allow the crabs to feed or to change location. In the fourth year when the crabs are 60-97 mm the pods merge and form large piles of crabs. These have been observed to comprise up to 500 000 crabs (Powell and Nickerson 1965b, Dew 1990).
At the larva stage the king crab is subject to predation by a number of planktivorous fish (Livingston et al. 1993, Wespestad et al. 1994). After settlement the king crabs are preyed upon by various fish. As the crab increases in size, the numbers of potential predators decrease although in a soft shelled state the crabs are more vulnerable (Gray 1964, Jewett 1978, 1982, Livingston et al. 1986, Loher et al. 1998). Other organisms that are believed to feed on juvenile king crab include horse crabs, sculpins and may be octopus (Powell and Nickerson 1965b). Sea otters have been observed feeding on mature king crab (Feder and Jewett 1981).
Prey of king crab larvae includes diatoms, nauplii, copepods and copepoids (Kurata 1960, Paul et al. 1979, Abrunhosa and Kittaka 1997). After settlement the diet of the crab consists of a range of benthic organisms including polychaetes, crustaceans, molluscs, echinoderms, ascidians and fish (Tarvierdieva 1979, Feder and Jewett 1981, Rafter 1996).
The king crab does not feed to capacity, but browses slowly or intermittently (Cunningham 1969). The smallest size groups (98-120 mm CL) had the highest feeding index and highest intestinal fullness. King crabs feed alternatively as active predators and filter feeders. They capture food by seizure and tear apart larger animals or scoop up and filter out small invertebrates and microfauna from the substratum. Filtering of the substratum could lead to accidental intake of sand and minute infaunal animals. The majority of the stomachs of the crabs in the study contained 3-4 of the major animal groups simultaneously and
occasionally algae (Cunningham 1969). A number of studies suggest that the king crab is an opportunistic feeder feeding on the most available benthos (Cunningham 1969, Feder and Paul 1980). Dietary studies of king crab have to a large extent been qualitative. The diet table below illustrates the diversity of food organisms king crab has been found to prey on (Table 1).
Table 1. Food items found in stomach of king crab. P (present) marks items found.
Reference
McLaughlin and
Hebard 1961 Bright 1967 Cunningham 1969 Tarverdieva 1976 Tarvierdieva 1979
Feder and Paul
1980 Feder et al. 1980
Jewett and Feder
1982 Jewett et al. 1989 Rafter 1996 Gerasimova 1997
Area of study
Southeastern
Bering Sea Cook Inlet, Alaska
Bristol Bay, Bering Sea
Bristol Bay, Bering Sea
Bristol Bay, Bering
Sea Cook Inlet, Alaska Southeastern Bering Sea
Kodiak Island, Alaska
Norton Sound, Alaska
Varanger fjord, Barents Sea
Varanger fjord, Barents Sea
Size of crab Food component
Size range 63-
193mm CL Juveniles Adults
shell width 80- 130mm
shell width more than 130
armour width: 126- 184mm
CL 2.89-5.04 (post larval king crab)
Mainly large crabs
(90-100mm CL) CL 32-201mm <150mm (CW) >150mm (CW)
Foraminifera P P P P P P P
Coelenterata P
Hydrozoa P P P P P P P P P P P
Polychaeta P P P P P P P P P P
Fam. Lumbrineridae P
Fam. Oweniidae P
Fam. Pectinariidae P
Fam. Polynoidae 3 P
Fam. Sabellidae P
Travisia sp. P
Sternaspis scutata P
Siphunculoidea (class) P P P
Priapuloidea (class) P P
Unid. Polychaeta P
Polychaete setae P
Nematoda P
Sipunculoidea P
Mollusca P P P P P P P
Bivalvia P P P P P P P
Fam. Cardiidae P
Cardiomya sp. P
Clinocardium ciliatum P P
Clinocardium sp. P
Cyclocardia crebricostata P
Liocyma fluctosa P
Macoma spp. P
Macoma spp. P P
Mya spp. P
Pandora spp. P
Serripes groenlandicus P
Spisula polynyma P
Tellina nuculoides P
Fam. Glycymerididae
Glycymeris subobsoleta P
Fam. Mytilidae P P P
Musculus sp. P
Modiolus Modiolus P
Fam. Nuculanidae P
Nuculana fossa P
Yoldia sp. P
Lyonsia sp. P
Crenella decussata P
Fam. Pectenidae P
Chlamys spp. P
Fam. Nuculidae P P P
Nucula tenuis P
Fam. Veneridae P
Unid. Bivalvia P P
Gastropoda P P P P P P P
Fam. Buccinidae
Neptunea lyrata P
Fam. Naticidae
Cylichana alba P
Polinices spp. P
Table 1. continue.
Reference/ Food component
McLaughlin and
Hebard 1961 Bright 1967 Cunningham 1969 Tarverdieva 1976 Tarvierdieva 1979
Feder and Paul
1980 Feder et al. 1980a
Jewett and Feder
1982 Jewett et al. 1989 Rafter 1996 Gerasimova 1997
Fam. Trochidae P
Margarites sp. P
Solariella spp. P P
Fam. Turridae
Oenopota spp. P
Gastropod eggs P P
Unid. Gastropoda P P
Scaphopoda P P
Crustacea P P P P
Order Amphipoda P P P P P P P
Photis spaskii P
Balanus spp. (U.Klasse. Cirripedia) P P P P P P
Calanoidea (copepods)
Chionoecetes bairdi (tanner crabs) P P
Cumacea P
Decapoda P P P P P P
Halacaridae (sea mites) P
Harpacticoida P
Isopoda P P
Majidae (spider crabs) P
Ostracoda (mussel shrimp) P P
Paguridae P P P P
Pandalus spp. (U. orden Caridea) P P
Pinnixa occidentalis P
P. camtschatica P P
Unid. Crustacea P P P P P
Echiurida
Echiurus echiurus P
Echinodermata P P P P P P
Asteroidea P P P P P P P P P
Pycnopodia helianthoides P
Echinoidea P P P P P P
Ophiuroidea P P P P P P P P
Strongylocentrotus droebachiensis P P P P
Echinarachnius parma P
Holothuroidea P P
Unid. echinoidea P
Unid. echinodermata P P
Other echinoderms P
Pogonophora P P P
Bryozoa P P P P P
Flustrella P
Ascidiae P P P P
Tunicata P
Pisces, bones, scales P P P P P P P
Fish roe P P
Porifera P P P P
Diatoms P P P
Silicoflagellates P
Tintinnida (ciliates) P
Unid. Animal material P P
Remains of plants P P P P P P P P P P P P
Brown algae P
Red algae P
Digested weight P P
Unid. Material P P P P
Unid.tissue P
Unid. Org material P
Slime P
Sand P P P P P P
Nylon rope fiber P
1.3 BIOLOGICAL INVASIONS
There is a lack of consistency in the use of terminology concerning invasive species (Ehrlich 1986, Williamson and Fitter 1996a, IUCN 2000, Lawrence 2000, Mack et al.
2000, Manchester and Bullock 2000, Prieur-Richard and Lavorel 2000, Hopkins 2001).
Invasion has been defined as the whole process from arrival of a new species into a community to establishment, maintenance and further spread (Prieur-Richard and Lavorel 2000). Mack et al. (2000) differentiate between immigrants and invaders where the latter have become abundant and widespread. In addition invaders were defined as having a negative effect on the environment. Although invasive species are associated with the above, this introduces a large degree of subjectivity to the term. Manchester and Bullock (2000) for example, argue that introduced species can have negative, positive and no impact upon native biota.
Biotic invasives and their descendants have been referred to as alien, non-indigenous, non- native, foreign, exotic, adventive, introduced, transferred, transplanted and introduced (IUCN 2000, Mack et al. 2000, Hopkins 2001). A native, or indigenous, species is defined as a species, subspecies or lower taxon occurring within its natural past or present range and dispersal potential (IUCN 2000). All these terms will be used interchangeably. Table 2 shows definitions of terms related to different levels of invasion success modified from Williamson and Fitter (1996b).
Table 2. Definitions and terms related to levels of invasion success. (Modified from Williamson 1996, Wiliamson and Fitter 1996b, Manchester and Bullock 2000).
Imported -brought into the country, contained.
Introduced -found in the wild, feral, casual, released but not breeding successfully, population not necessarily self-maintaining.
Established -with a self-sustaining population, naturalized, feral and breeding successfully, released and breeding successfully.
Pest -with a negative economic effect.
Escaping -transition from imported to introduced.
Establishing -transition from introduced to established.
Becoming a pest -transition from established to pest.
Biological invasions are not a new phenomenon or solely human induced. But through increased mobility man has significantly extended the geographic scope, rate and numbers of species involved (di Castri 1989). Today biological invasions are considered major agents of global change and one of the main threats to marine systems. Although most invaders have minor consequences those that do succeed may have economic,
environmental and ecological effects (Elton 1958, Vitousek 1990, Williamson 1996, Vitousek et al. 1996, Mack et al. 2000, Hopkins 2001, ICES 2001).
The three major categories for introductions include accidental introductions, species imported for a limited purpose that escape, and deliberate introductions (Levin 1989).
Digging of canals, accidental transport on ships and deliberate introductions are important vectors causing change in the distribution of species in the ocean (Carlton and Geller 1993, Minchin and Gollasch 2002).
King crab is only one of many species that have been introduced for economic reasons.
Today many introductions are the foundation of important industries in their new area, while others have proven disastrous. Releases of exotic species have caused problems such as pests, pathogens and weeds in forestry, agriculture and aquaculture as well as
endangered and caused extinction of native species. Introduction of exotics is regarded as one of the largest threat to biodiversity (Elton 1958, Soulé 1990, McNeely 2001).
Economic damages due to biological invasions can be very large. The damage can be both due to interspecific interaction between the exotic and native species and through effects on ecologically important species, habitats or ecosystems (Williamson 1996, Mack et al.
2000). There have been attempts to measure the costs of biological invasion as well as the value of biodiversity for comparison with alternative use of the resource. This is a task that is difficult, many will say impossible, and full of controversies. It could nevertheless be used as a tool to shed some light on the costs and benefits involved (Barbier 2001, Nunes and van den Bergh 2001). It is clear that the cost of introductions can be great and in many instances it is not carried by those who benefit from the exotics. This can be a source of conflict (Elton 1958, Ewel et al. 1999, Mack et al. 2000, McNeely 2001). In many
instances decisions concerning introductions are ultimately a political issue of economic gains and public emotions versus the value of preventing biological damage (Soulé 1990, Jenkins 1999, Mack et al. 2000).
Invasive species can be fought at three stages with each stage being more difficult and costly. Invaders can be prevented from entering a new area, eradicated when first detected or attempts can be made to control their population size once established (Elton 1958, Mack et al. 2000). Control, management and eradication programs for exotic species can be very controversial and involve many competing interests. These programmes may involve destructive mechanical and chemical methods as well as biological control agents. The latter could result in the release of another pest (Elton 1958, Mack et al. 2000). Invaders are rarely eradicated once established and the success of control is dependent on commitment rather than the tools used. Long term, ecosystem wide strategies have proven the most effective (Mack et al. 2000).
Exotics may do well in a new environment due to the lack of predators, competitors, parasites and/or abundance of spatial and dietary resources as well as more favourable abiotic conditions. Behavioural traits may also determine the success of invasives. Many invasions are facilitated by human-caused disturbance of native communities (Williamson and Fitter 1996b, Holway and Suarez 1999, Mack et al. 2000, Hopkins 2001).
Based on Vitousek (1990) Crooks (2002) identified three major effects of exotics on ecosystems. These include the alteration of flow, availability and quality of 1) nutrient resources within biogeochemical cycles, 2) trophic resources within food webs and 3) physical resources. Through predation, competition or breaking links in the system, invasive species can cause extinction or stress native species. The result can be a system with simpler and less sustainable community structure. Reduced biodiversity has also been found to lead to higher vulnerability to pests and diseases (Begon et al. 1996, Sakai et al.
2001). A number of introductions have caused major structural changes in food web structures. Examples include the impact of zebra mussel (Dreissena polymorpha) on freshwater systems in Europe and North America (Strayer et al. 1998, Aldridge et al. in
press), the Nile perch (Lates nioticus) on the Lake Victoria ecosystem (Moreau et al. 1993) and the opossum shrimp (Mysis relicta) in freshwater lakes in the United States and Canada (Spencer et al. 1991).
A number of crustaceans have occupied new areas (ICES 2001) among them the European shore crab. The European shore crab (Carcinus maenas) originally from the European Atlantic waters is now established on the Atlantic and Pacific coasts of U.S.A, in Brazil, Panama, Hawaii, Ceylon, Australia and South Africa. It is known as an aggressive predator controlling its prey populations both in its native and introduced areas. In South Africa the crab is restricted to sheltered localities which make up a small part of the coast. The crab is not expected to compete with or displace indigenous crab species as dietary and habitat preferences differentiate. It could, however, be a threat to South African lagoon systems where many important mariculture centres and conservation areas are situated. In North America crab invasion has been followed by a decline of benthic invertebrates and
shorebird species. It is also a pest of commercial bivalve culture operations (Le Roux et al.
1990, Griffits et al. 1992, Grosholz and Ruiz 1996).
Only a small number of studies have attempted to quantify the impacts of marine invasive species on the native biota. Even fewer have measured the effects on multiple trophic levels. Grosholz et al. (2000) monitored a Californian coastal system over 9 years to determine the impacts of the nonindigenous Carcinus maenas on the marine food web. It was found that the crab had both direct and indirect effects on the abundance of a number of native species.
Knowledge of the ecological impact of introduced species in marine systems is limited compared to the understanding of their effects on terrestrial and freshwater communities (Grosholz et al. 2000, ICES 2001). Studies of marine invasions usually have poor
predictive power as they seldom combine wide-ranging descriptive data and quantitative or experimental results. The ability to predict which species will have serious impacts is important to understand ecosystem structure and functioning and aid in management and control efforts (Grosholz and Ruiz 1996).
1.4 ECOPATH AND ECOSIM
The Ecopath with Ecosim (EwE Version 5) software is a tool for constructing a model of trophic flows between compartments in an ecosystem. Ecopath offers a network analysis which can provide information on the structure and functions of ecosystems (Wulff et al.
1989, Christensen and Walters in press). Ecopath also provides a mixed trophic impact routine that quantifies all direct and indirect trophic effects by summing the negative and positive impacts for each group (Christensen 1995a).
Ecopath is a mass-balance approach describing an ecosystem for a given period of time.
Ecosystem changes can be simulated over time using Ecosim, which is a dynamic
ecosystem model. Ecosim can be used to explore the dynamics of the system as well as the responses of the system to various fishing patterns and environmental disturbances
(Walters et al. 1997, Pauly et al. 2000).
The Ecopath model assumes that for any producer at the time period considered consumption can be described by
Consumption = production + non-assimilated food+ respiration 1) Production is estimated from
Production = predation mortality + non-predation mortality + net migration +
biomass accumulated 2)
Predation mortality (equation 2) can be estimated as consumption by all predators and thus links predators and prey.
Ecopath requires three of the following four input parameters for each of the functional groups in the model: biomass, production/biomass, consumption/biomass and ecotrophic efficiency. Ecotrophic efficiency (EE) expresses the proportion of production of a given group that is used for predation in the system. EE has a scale from 0 to 1 where 1 is complete utilization by other species.
EE = 1 – (non-predation mortality / production rate)
Ecopath sets up as many linear equations as there are groups in the system and solve for unknown values. The EE parameter is the main tool used to balance the model in order to assure that no group is being preyed upon beyond their level of production. Other
parameters important to evaluate the model are production/consumption ratio and food electivity (Christensen et al. 2000).
Additional parameters to those listed above needed as input in the Ecopath model include diet composition, assimilation rate, net migration rate, biomass accumulation rate as well as fisheries catch and discards. The data input required to construct the model has commonly been collected in fisheries analysis. By combining these data into one coherent picture the major predator-prey relationships are highlighted. An Ecopath analysis can also help identifying critical data gaps in the knowledge of the ecosystem of concern (Christensen and Pauly 1992, Christensen et al. 2000).
The basics of Ecosim are derived from the Ecopath equation and consist of biomass dynamics expressed in the form of coupled differential equations. It takes into account the trade-off between searching for prey and being exposed to predators. This can be
manipulated through changing the vulnerability parameter of prey. The vulnerability parameter ranges from 0 (prey not vulnerable, implying bottom-up control) to 1 (prey vulnerable, implying top-down control). Ecosim requires input of life history parameters and allows for linking of juvenile and adult groups to better represent ontogenetic shifts (Pauly et al. 2000).
2. MATERIALS AND METHODS
2.1 THE SØRFJORD MODEL
An Ecopath model by Pedersen, T., Nilsen, M., Nilssen, E.M. and Berg, E. (unpubl.) on the Sørjord system was used as a base model representing a North Norwegian fjord. The model represents an average year and is based on sampling between 1993 and 1996. The Ecopath input and output parameters for the functional groups in the Sørfjord model (Model I) are shown in Table 3, while the diet matrix can be found in Appendix A.
Table 3. Input and output parameters for the Sørfjord model without king crab (Model I).
Parameters summarised include trophic level as well as annual biomass, production per biomass (P/B), consumption per biomass (Q/B), ecotrophic efficiency (EE), biomass accumulation
(Biomass accum.), assimilation coefficient (Assim. coef.), production per consumption (P/Q) and harvest by cod fleet. Input parameters are shown in black while values estimated by the model are marked in blue.
Group name
Trophic level
Biomass (t/kmý)
P/B (/year)
Q/B
(/year) EE
Biomass accum.
Assim.
coef. P/Q Harvest (t/kmý) Cormorants 4.32 0.0009 0.125 37.10 0.533 0.0 0.2 0.003 0.00006 Mammals 4.24 0.01 0.102 35.30 0.490 0.0 0.2 0.003 0.0005 Large cod 3.35 1.81 0.42 3.00 0.869 0.1 0.2 0.140 0.370 Small cod 3.36 0.14 1.70 6.00 0.384 0.0 0.2 0.283
Large other fish 3.08 0.78 0.50 3.00 0.631 0.0 0.2 0.167 0.150 Small other fish 3.14 0.575 1.70 6.70 0.900 0.0 0.2 0.254
Herring 3.07 0.22 1.00 6.00 0.900 0.0 0.2 0.167 Euphausiids 2.11 4.515 2.50 16.70 0.900 0.0 0.3 0.150 Small zooplankton 2.05 20.0 6.50 26.00 0.359 0.0 0.3 0.250 Schypomedusae 3.10 0.72 6.50 17.33 0.133 0.0 0.2 0.375 Chaetognaths 3.05 0.20 3.80 19.00 0.493 0.0 0.2 0.200 Shrimp 2.59 0.193 2.00 13.30 0.900 0.0 0.2 0.150 Other large zooplankton 2.00 0.706 2.00 13.30 0.900 0.0 0.2 0.150 Large decapoda 2.93 0.363 0.50 3.33 0.900 0.0 0.2 0.150 Predatory benthos 2.89 1.273 0.50 3.33 0.900 0.0 0.2 0.150 Detrivore polychaetes 2.00 43.0 0.74 4.93 0.105 0.0 0.2 0.150 Small benthic crustaceans 2.12 4.0 0.50 3.33 0.433 0.0 0.2 0.150 Small molluscs 2.08 26.0 0.35 2.33 0.634 0.0 0.2 0.150 Large bivalves 2.00 62.9 0.19 2.11 0.084 0.0 0.2 0.090 Detrivore echinoderms 2.00 41.0 0.20 2.22 0.284 0.0 0.2 0.090 Other benthic invertebrates 2.00 2.0 0.50 3.33 0.243 0.0 0.2 0.150 Phytoplankton 1.00 20.0 60.00 - 0.460 0.0 - Detritus 1.00 50.0 - - 0.493 -
Sørfjord (69°40’N, 19°40’E) is the inner part of the Ullsfjord-Sørfjord system and is situated in Troms County, Northern Norway (Figure 2). The fjord is about 27 km long with a maximum with and depth of 3 km and 130 m respectively, and covers an area of 55 km2. It is separated from Ullsfjord by a 300 m wide and 8 m deep sill (Eliassen and Eilertsen 1988). The fjord consists of a well mixed outer basin (max. depth 125 m) with winter and summer temperatures of 3 °C and 9 °C, a shallow mid-fjord basin of 65 m depth and an inner basin with a maximum depth of 130 m. The water column of the two latter areas is stratified during summer with water temperatures below the thermocline of 3 °C, or lower, in winter and a maximum of 6 °C in autumn (Kanapatihippillai et al. 1994).
Figure 2. Map of the Sørfjord-Ullsfjord system.
2.2 CONSTRUCTION OF THE SØRFJORD MODEL WITH KING CRAB
A literature study was conducted to identify predators and prey of the king crab as well as other life history parameters relevant for the model. Three life history stages were
identified: planktonic, juvenile and mature stage. The planktonic stage was omitted from the model as this stage was not included for the other groups in the Sørfjord model. In addition Ecosim does not allow for linking of more than two groups.
Two functional groups, large and small king crab, were introduced into Model I. In order to emphasise the possible trophic impacts the king crab may have on the ecosystem, their biomasses were increased to the maximum level where it could be avoided that ecotrophic efficiency (EE) of any group became larger than unity. The largest biomass of king crab obtained for the model to balance was 2.8 t/km y-1 and 1.2 t/km y-1 for large and small king crab respectively. The model was run under the assumption that the predators of king crab feed upon them according to relative biomass available. This gave very low electivity values of cod and other fish as predators on small king crab. Feeding electivity values are output of Ecopath expressing the food preference of consumers. Electivity values range from -1 (total avoidance) to 1 (exclusive feeding) (Christensen et al. 2000). The food preference of the fish groups for small king crab was much lower than for large decapods.
The proportion of small king crab in the diets of these groups was therefore adjusted up so that electivity values resembled those of large decapods. When balancing the model it was noted that it was very sensitive to changes in diet input. A 0.01 increase in the proportion of large king crab in diet of large cod, for example, increased EE of large king crab about 20%
from 0.512 to 0.607. The balanced Ecopath model for Sørfjord model with king crab (Model II) is shown in Table 4 and the diet matrix in Appendix B.
Table 4. Input and output parameters for Sørfjord model with king crab (Model II). Parameters summarised include trophic level as well as annual biomass, production per biomass (P/B),
consumption per biomass (Q/B), ecotrophic efficiency (EE), biomass accumulation (Biomass accum.), assimilation coefficient (Assim. coef.), production per consumption (P/Q) and harvest by cod and king crab fleets. Input parameters are shown in black while values estimated by the model are marked in blue.
Group name
Trophic level
Biomass (t/kmý)
P/B (/year)
Q/B (/year) EE
Biomass accum.
Assim.
coef. P/Q
Harvest (t/kmý) Cormorants 4.36 0.0009 0.125 37.10 0.533 0.0 0.2 0.003 0.00006 Mammals 4.19 0.01 0.102 35.30 0.490 0.0 0.2 0.003 0.0005 Large cod 3.40 1.81 0.42 3.00 0.789 0.1 0.2 0.140 0.370 Small cod 3.41 0.14 1.70 6.00 0.333 0.0 0.2 0.283
Large other fish 3.11 0.78 0.50 3.00 0.553 0.0 0.2 0.167 0.150 Small other fish 3.16 0.525 1.70 6.70 0.900 0.0 0.2 0.254
Herring 3.07 0.22 1.00 6.00 0.900 0.0 0.2 0.167 Euphausiids 2.11 4.217 2.50 16.70 0.900 0.0 0.3 0.150 Small zooplankton 2.05 20.0 6.50 26.00 0.356 0.0 0.3 0.250 Schypomedusae 3.10 0.72 6.50 17.33 0.133 0.0 0.2 0.375 Chaetognaths 3.05 0.20 3.80 19.00 0.493 0.0 0.2 0.200 Shrimp 2.59 0.178 2.00 13.30 0.900 0.0 0.2 0.150 Other large zooplankton 2.00 0.691 2.00 13.30 0.900 0.0 0.2 0.150 Large decapoda 2.94 0.432 0.50 3.33 0.900 0.0 0.2 0.150 Predatory benthos 2.89 1.658 0.50 3.33 0.900 0.0 0.2 0.150 Detrivore polychaetes 2.00 43.0 0.74 4.93 0.247 0.0 0.2 0.150 Small benthic crustaceans 2.12 4.0 0.50 3.33 0.638 0.0 0.2 0.150 Small molluscs 2.08 26.0 0.35 2.33 0.918 0.0 0.2 0.150 Large bivalves 2.00 62.9 0.19 2.11 0.351 0.0 0.2 0.090 Detrivore echinoderms 2.00 41.0 0.20 2.22 0.790 0.0 0.2 0.090 Other benthic invertebrates 2.00 2.0 0.50 3.33 0.460 0.0 0.2 0.150
Large king crab 3.03 2.8 0.20 3.00 0.512 0.0 0.2 0.067 0.010 Small king crab 3.04 1.2 1.00 5.00 0.459 0.0 0.2 0.200
Phytoplankton 1.00 20.0 60.00 - 0.457 0.0 - Detritus 1.00 50.0 - - 0.497 -
A short description of the functional groups included in the Sørfjord model including king crab (Model II) follows. Details on the parameters used in the original Sørfjord model can be found in Pedersen, T., Nilsen, M., Nilssen, E.M. and Berg, E. (unpubl.).
1. Cormorants
Cormorants (Phalacrocorax carbo carbo) are not expected to feed on or be fed upon by king crab.
2. Mammals
This group includes harbour porpoises (Phocoena phocoena), harbour seals (Phoca vitulina), Eurasian otters (Lutra lutra) and harp seals (Phoca groenlandica Erxleben).
Harbour porpoises feed on pelagic or semidemersal fishes including herring, capelin, mackerel, sardines, cods and whiting (Tomilin 1967, Rae 1973). Harbor seals have been found to eat fishes, octopus and crustaceans including Idotea baltica spp. and Thysanoessa spp. (Berg et al. 2002). Pacific sea otters (Enhydra lutris) have been observed feeding on mature king crab (Feder and Jewett 1981, Fukuhara 1985). Harp seals feed on herring, cod and pelagic crustaceans including Thysanoessa spp., Parathemisto libellula, Pandalus spp., Crangon spp. and Sabinea septemcarinatus (Lindstrøm 1998, Nilssen et al. 1992, 1998).
King crabs have been observed to be missing legs in areas where seals are common along the North Norwegian coast. This suggests that the king crab is subject to seal predation (Nilssen, E. Norwegian College of Fishery Science, personal communication).
The mammal group will, on the whole, be treated as feeding on large king crab. Under the assumption that king crabs are fed upon according to relative biomass, king crab proportion in the diet of mammals was put to 0.346.
3 and 4. Adult and juvenile cod
As the majority of cod preyed upon are smaller than 35cm this length was used to divide the cod group into small and large cod. Cod in Sørfjord feed on crustaceans including amphipoda and Hyas spp (Kanapathippillai et al. 1994). Pacific cod are important predators of soft-shell red king crab (Jewett 1978, Fukuhara 1985, Livingston et al. 1986, Livingston 1989). Smaller crustaceans have been found to be more common in small cod while cod larger than 60 cm gradually shifts to a mixed diet of larger prey, primarily fishes (Daan 1973, Livingston et al. 1986). Livingston (1989) found that cod larger than 60 cm
contained whole red king crab (CL 53-160 mm) more often than cod with lengths of 30-59 cm. The percentage of weight of king crab legs in cod diet was generally less than 25%.
The adult cod group prey upon both large and small king crab, but is not preyed upon by king crab. Small cod are less than 35 cm and will have similar diet as large cod, but restricted to smaller organisms. This group is expected to prey upon small king crab.
5 and 6. Large and small other fish
This group consists of haddock (Melanogrammus aeglefinus), long rough dab (Hippoglossoides platessoides), plaice (Pleuronectes platessa), whitch flounder (Glyptocephalus cynoglossus), wolffish (Anarhichas lupus), redfish (Sebastes spp.),
whiting (Merlangius merlangius) and saithe (Pollachius virens). Large and small other fish are longer and shorter than 35 cm respectively.
In this group only wolffish has been observed to feed on adult king crab in aquarium (Gerasimova 1997, personal communication Nilsen, M. Norwegian College of Fishery Science). In its native range the king crab has a number of fish predators. Yellowfin sole (Limanda aspera) is thought to be an important predator of zoea and megalops of king crab so are other flatfish including rock sole (Lepidopsetta bilineata) and flathead sole
(Hippoglossoides elassodon). Halibut (Hippoglossus stenolepis), sablefish (Anplopoma fimria), eelpout (Lycodes palearis), skates (Raja spp.), sculpins (Hemilepidotus
hemilepidotus, Myoxocephalus spp.), snailfish (Liparis spp.), sockeye salmon
(Oncorhynchus nerka) and Walleye pollock (Theragra chalcogramma) are also potential predators of various stages of king crab (Gray 1964, Healey 1980, Jewett 1982, Haflinger and McRoy 1983, Wespestad et al. 1994, Loher et al. 1998). King crabs of three years and older are too large for most fish to feed on (Jewett and Powell 1981).
Capelin is one of the fish species that has been found in king crab stomachs (Feder and Jewett 1981). There is not agreement in the literature if fish found in king crab stomachs have been eaten alive or dead. Cunningham (1969) found it unlikely that food organisms had been eaten dead. This is supported by the findings of Logvinovich (1945, as in
Cunningham) who rejected that the crabs are scavengers as they did not accept putrefied food organisms during laboratory experiments. McLaughlin and Herbard (1961), on the other hand, found decomposed organisms in crab stomachs indicating that crabs had been feeding upon dead material. Stone et al. (1993) discovered that individual king crabs periodically returned to a cleaning station for local fishermen in Auke Bay. They suggested that the crabs were attracted to this location by the periodic disposal of offal. The crab has also been observed feeding on carcasses of fish (Zhou and Shirley 1997). Bright (1967) fed king crab dead flounder, but the crab did not attempt to catch live fish. Fish is also used successfully as bait in king crab pots.
As a group the “other fish” are likely to prey upon king crab. Large king crab will only be fed upon by large other fish. Due to the relatively high mobility of fish the king crab will not be feeding on fish in the model.
7. Herring
Herring (Glupea harengus) may feed on planktonic king crab larvae, but in this model the larval stage of the king crab is excluded.
8. Euphausiids
Thysanoessa inermis and Thysanoessa raschii are the main euphausiid species found in Sørfjord. Prey found in the stomach of king crab zoeae captured in nature includes diatoms, barnacle nauplii and crab larvae (Bright 1967). Early life stages of the euphausiid group are likely to be preyed upon by king crab larvae, but in the model the larvae stage is excluded.
9. Small zooplankton
Small zooplankton (<8 mm) are mainly herbivorous copepods, cladocera, ciliates, rotifers and appendicularians. Copepods and copepodids have been used as food in experiments with king crab zoeae. At sufficiently high concentrations the zoeae will consume up to 12 copepods per day (Paul et al. 1979). Copepods have been found in adult king crab stomachs (Jewett and Feder 1982, Jewett et al. 1989). In the model there will be no direct interaction
between king crab and small zooplankton as the king crab larvae stage is ignored and the findings of copepods in the adult is not considered significant.
10. Schypomedusae
The common schypomedusae in Sørfjord are Cyanea capillata and Aurelia aurita. Jellyfish could be eating planktonic king crab stages. Coelenterates have been found in adult king crab stomachs (Tarverdieva 1976, 1979, Feder and Paul 1980, Feder et al. 1980, Jewett et al. 1989, Rafter 1996, Gerasimova 1997). The stage of the coelenterates found is not
specified. Assuming the crab feed on the bottom living stage the schypomedusae group will not prey or get preyed on by the king crab stages in this model.
11. Chaetognaths
The dominating species is the carnivorous Sagiita elegans that could feed on planktonic king crab. In the model this group will not interact directly with king crab.
12. Shrimp
Pandalus borealis is the dominant species, while Eualus gaimardii, Eualus pusiolus, Spirontocaris spinus, Pontophilus norvegicus, Crangon crangon and Pandalus montagui are also present. Pandalus spp. have been found in king crab stomachs (Feder and Paul 1980, Jewett and Feder 1982), but due to their mobility they are unlikely to be important prey of king crab.
13. Other large zooplankton
This group consists of zooplankton larger than 8 mm and include pelagic amphipods, mysidae and pelagic polychaetes. These could prey and get preyed upon by larval king crab, but feeding interaction with the king crab groups in the model is unlikely.
14. Large decapods
This group is made up by Brachyurans, mainly Hyas areneus, Hyas coarctatus and Carcinus maenas, and Anomurans, which include hermit crabs and Munida spp.
Crustaceans of this category are found in king crab stomachs (McLaughlin and Hebard
1961, Tarverdieva 1979, Feder et al. 1980, Feder and Paul 1980, Jewett and Feder 1982, Jewett et al. 1989, Rafter 1996, Gerasimova 1997).
15. Predatory benthos
Predatory molluscs and asteroids, Actinaria and free living errante polychaetes are included in predatory benthos.
16. Detrivore polychaeta
Important taxa in this group are Terebellidae and Flabelligerida. They are sedentary polychaetes that feed on detritus.
17. Small benthic crustaceans
Amphipods, mysiids, cumaceans and other hyperbenthic groups are included in this group.
Due to their small size they will not feed on king crab in the model.
18. Small mollusca
This group of detrivores and herbivores includes gastropoda, polyplachophora, small bivalves and scaphopoda.
19. Large bivalves
Large bivalves are large, long lived animals with low mortality rates. Species included are Artica islandica, Musculus niger and Chlamys islandica.
20. Detrivore echinoderms
Detrivore echinoderms are largely made up by Ctenodiscus crispatus, but also Ophiopholis aculeate and Ophiura species.
21. Other benthic invertebrates
This group comprises Priapulidae, Hirudinea, Pycnogonidae, Brachiopoda, Ascidia and sea urchins (Echinodea).
22 and 23. Large and small king crabs
Large king crabs are crabs of 5 years and older while small king crabs are younger than 5 years old. At 5 years they are about 100 mm carapace length (CL) and have attained sexual maturity (Powell and Nickerson 1965a, Otto et al. 1989, Rafter 1996). At this stage it is assumed that the predator prey relationship of the king crab changes due to the size attained as well as change in habitat. It is also assumed that the diet and mortality rate does not differ between the sexes. Relative proportion of small to large crabs was calculated to 30:70 based on length and age data on king crab from Varanger fjord and literature study (Nilssen and Sundet in prep.). Calculations are shown in Appendix C.
Production per biomass (P/B) for king crab
The P/B values used in other models for king crab or groups comparable to king crab show great variations.
Bundy et al. (2000) used a P/B of 0.282 yr-1 for large crustaceans in an Ecopath model of the Newfoundland-Labrador Shelf. This value was estimated from catch and biomass data on American lobster, snow crab and various non-commercial species. In the Ecopath model of the Northern Gulf of St. Lawrence P/B for snow crab is 0.251 yr-1 and takes into account the high mortality of young and low mortality of old males (Morisette 2001). P/B values for various crabs in a study conducted in Chile ranged from 0.5 yr-1 to 1.95 yr-1 (Ortiz and Wolff 2002). A P/B of 0.6 yr-1 for king crab in the Eastern Bering Sea was suggested in Trites et al. (1999), while snow and tanner crabs were given P/B of 1 yr-1. All P/B values were based on the Tanner crab (Paul and Fuji 1989).
P/B as estimated by the model of Brey (1999) for small and large king crab was calculated to 0.232 yr-1 and 0.114 yr-1 at 5°C. The value for small king crab is quite low both
compared to the P/B of large decapods in the Sørfjord model and to the other values found in the literature for crab.
Based on the information listed above a P/B value of 1 yr-1 was chosen for small king crab and 0.2 yr-1 for large king crab.
Consumption per biomass (Q/B) for king crab
Q/B values for various crabs in a Chilean Ecopath model range from 4.5 yr-1 to 9.9 yr-1 (Ortiz and Wolff 2002). Morisette (2001) used a value for snow crab of 1.3, while Bundy et al. (2000) used a value of 5.9 for lobsters.
Based on daily ration of adult male king crab (mean weight 2144 g) of 0.31% of their body weight (Tarvierdieva 1979) a Q/B of 1.132 yr-1 was calculated. Gerasimova (1997)
collected information from Russian studies of food digestion and diurnal ration of king crab. A crab of 362 g has a daily ration of 1.5% of body weight giving a Q/B of
[(5.43/362)*365] 5.48 yr-1. A crab of 2710 g has a daily ration of 0.32% of body weight resulting in Q/B of 1.17 yr-1, while crab of 4091 g of daily ration 0.15 yr-1 has a Q/B of 0.54 yr-1.
Q/B values calculated based on a feeding and growth study of king crab by Zhou et al.
(1998) were significantly higher than values found in other literature. They were as follows:
Ovigerous females of about 1000 g: (59.25 g food/1000 g crab)*365 days = 21.63 yr-1. Juvenile females of about 530 g: (31.5 g food/530 g crab)*365 days = 21.7 yr-1. Males of about 900g: (50.85 g food/900 g crab)*365days = 20.62 yr-1.
A rearing study by Rice et al. (1985) found juvenile crabs (3-4 cm CL) to consume 0.011 g herring per day per gram crab. This gives a Q/B value of (0.011*365) 4.015 yr-1. Feeding rate of 6.3 mm (±0.1 mm SE) king crabs was found to be 0.081 g (±0.0076 g SE) per day per gram crab in experiments by Molyneaux and Shirley (1988), giving a Q/B of
(0.081*365) 29.565 yr-1.
The Q/B for king crab in Aydin et al. (2002) was 5 yr-1 in Eastern Bering Sea and is based on Trites et al. (1999) whose value is based on Tanner crab. An estimate of Q/B in the Western Bering Sea, on the other hand, is based on adult king crab (>80 mm CL) and has a value of 2.27 yr-1. Juvenile king crab (40-80 mm CL) Q/B has been estimated to range from
3.5-5.0 yr-1. The range is due to seasonal study differences (Aydin et al. 2002, Aydin, K.Y., Alaska Fisheries Science Center, personal communication).
Based on the literature study a Q/B value of 3 yr-1 for large king crab and 5 yr-1 for small king crab were chosen.
Predators and prey of king crab
The literature study revealed that king crab feed on a wide range of organisms (Table 1) and was used to determine prey groups of king crab in the Sørfjord model. An overview of the functional groups of Model II that the king crab feed or get fed upon is shown in Table 5. Cannibalism within small king crabs has been documented by Rounds et al. (1989) and Damsgård et al. (1997). Large king crabs in Norwegian waters have been found to contain juvenile king crabs (Haugan, T.A. Norwegian College of Fishery Science, personal communication).
The diet of the king crab has been suggested to reflect prey availability (Takeuchi 1959, Feder and Paul 1980). In the model it was assumed that the proportion of prey in diet of king crab is reflected by the relative availability, in terms of biomass, of the respective prey groups it feeds on. The proportion of king crab in the diets of its predators was put equal to relative biomass availability of king crab. The diets were then scaled to one.
24. Phytoplankton
Diatoms dominate during the spring bloom, while there is a low abundance of
dinoflagellates and coccolithophorids in Sørfjord. Diatoms are an important energy source for stage one king crab zoeae (Bright 1967, Paul et al. 1989). Silicoflagellates have been found in juvenile king crab (Feder et al. 1980).
25. Detritus
This group consists of dead organic material as well as bacteria. The king crab is expected to eat detritus when feeding by scooping up and filtering material. Since the model
